1. Field of the Invention
The present invention relates to particulates, preferably ultrafine particulates of titanium oxide suitable for ultraviolet shielding uses, photocatalytic uses and the like and a production process therefore. More specifically, the present invention relates to particulates, particularly ultrafine particulates of high rutile content titanium oxide obtained from titanium tetrachloride as a material by a vapor phase process.
2. Description of the Related Art
Particulates, particularly ultrafine particulates of titanium oxide have very wide application areas in the industrial field and their diversified uses include an ultraviolet-shielding material, an additive to silicone rubber, a photocatalyst and the like. The xe2x80x9ctitanium oxidexe2x80x9d is referred to as xe2x80x9ctitanium dioxidexe2x80x9d in Japanese Industrial Standard (JIS) but the term xe2x80x9ctitanium oxidexe2x80x9d is used as a common name. Accordingly, this simple term xe2x80x9ctitanium oxidexe2x80x9d is hereinafter used in the present invention.
The importance of titanium oxide is increasing in the use for shielding an ultraviolet ray, for example, in the field of cosmetics, clothing and the like. As a shielding material, ultrafine particulates of titanium oxide are being used in many cases because of its high safety. For the shielding, two functions of absorbing and scattering the ultraviolet rays are necessary. The ultrafine particulates of titanium oxide have both of the two functions.
Titanium oxide has three crystal forms, i.e., brookite, anatase, and rutile, latter two of which are very important for industry. And because the band gap (corresponding to excitation energy) of rutile is lower than that of anatase (i.e., the optical absorption wavelength range is on the longer wavelength side than anatase), rutile has been considered to be preferable for the ultraviolet-shielding use. However, in actual ultraviolet-shielding uses, scattering effect depending on particle diameter as well as to this absorption has to be coped with.
Recently, it has been reported that titanium oxide has a property of absorbing ultraviolet rays at a wavelength of about 400 nm or less to excite the electrons in the outermost shell, allowing the generated electrons and holes to reach the surface of particulates, where they combine with oxygen or water to generate various radical species, thereby decomposing organic materials that exist near the surface of the particle. Therefore, in the case of using titanium oxide in cosmetics and the like, generally it has been widely attempted to practice surface treatment on the surface of particulates, particularly ultrafine particulates of titanium oxide.
The fine particulates of titanium oxide are also used for making use of the photocatalytic reaction resulting from photoexcitation of titanium oxide. Furthermore, where titanium oxide is used for scattering ultraviolet rays, ultrafine particulates of titanium oxide having a primary particle size of about 80 nm are used. Generally, the primary particle diameter of ultrafine particulates has not been made clear. However, usually, those fine particulates having about 0.1 xcexcm or less are referred to as such.
The production process for titanium oxide is roughly divided into a liquid phase process where titanium tetrachloride or titanyl sulfate is hydrolyzed in a hydrophilic solvent and a vapor phase process where a volatile material such as titanium tetrachloride is vaporized and then the resulting vapor is reacted with an oxidizing gas such as oxygen and steam. In the vapor phase process, ultrafine particulate titanium oxide is obtained. However, only such titanium oxide as one composed of anatase as a main phase has been obtained. Therefore, conventionally, ultrafine particulate titanium oxide of a rutile structure has been obtained by a liquid phase process.
In general, the powder of titanium oxide produced by the liquid phase process disadvantageously undergoes heavy aggregation. For this reason, when titanium oxide is used in cosmetics and the like, the titanium oxide must be strongly cracked or pulverized, so that there arise problems such as mingling of abraded materials attributable to the pulverization treatment or the like, non-uniform distribution of the particle size, or bad touch feeling.
Several production processes for titanium oxide having high rutile contents have heretofore been proposed. For example, Japanese Patent Application Laid-Open No. 3-252315 discloses a production process where the ratio of hydrogen in the mixed gas comprising oxygen and hydrogen in the vapor phase reaction is changed to adjust the ratio of rutile content and a process for producing high purity titanium oxide having a rutile content of 99% or more by adjusting the concentration of hydrogen to from 15 to 17% by volume. Also, Japanese Patent Application Laid-Open No. 6-340423 discloses production process for titanium oxide having high rutile content (the rutile content being from 85% by weight to 90% by weight) where the production is performed by setting the molar ratio of titanium tetrachloride, hydrogen and oxygen in the mixed gas to specified mixing ratios.
In the case of titanium oxide produced by the vapor phase process, the same problems as in the production by the liquid phase process will arise. That is, although particulates, particularly ultrafine particulates of titanium oxide may be obtained by the conventional vapor phase process, only particulates of titanium oxide which have undergone grain growth can be obtained. Thus, for obtaining ultrafine particulates of titanium oxide, the titanium oxide must be strongly cracked or pulverized. Moreover, titanium oxide having high rutile content is ultrafine particulate, though ultrafine particulate, does not have sufficient specific surface area and it is insufficient in dispersibility, which is desired in various uses to start with cosmetics.
The present invention has been made to solve the above-described problems and an object of the present invention is to provide particulates, particularly ultrafine particulates of titanium oxide having a high rutile content which undergo considerably reduced aggregation and are highly dispersible.
Another object of the present invention is to provide a production process for producing such particulates, particularly ultrafine particulates of titanium oxide having a high rutile-content.
The present inventors have made extensive investigations with view to solving the above-described problems. As a result, they have found that particulate, particularly ultrafine particulate titanium oxide with a high rutile content having specified properties, which is titanium oxide having a high rutile content and a high BET specific surface area can be obtained by a vapor phase process comprising preheating a diluted titanium tetrachloride gas and an oxidizing gas, respectively, supplying them at specified flow rates into a reaction tube, and allowing them to react with each other for a specified time of residence at high temperatures. Thus, the present invention has been accomplished.
That is, the present invention relates to the followings:
[1] Particulate titanium oxide comprising a mixed crystal titanium oxide containing rutile crystal produced by a vapor phase process, wherein the titanium oxide has a property represented by the following general formula (1)
Rxe2x89xa71,300xc3x97Bxe2x88x920.95xe2x80x83xe2x80x83(1)
wherein R represents a rutile content (%) measured by an X-ray diffraction method and B represents a BET specific surface area (m2/g), which ranges from about 15 to about 200 m2/g.
[2] The particulate titanium oxide as described in 1 above, wherein the BET specific surface area represented by B is about 40 to about 200 m2/g.
[3] The particulate titanium oxide as described in 1 above, wherein the titanium oxide has a 90% cumulative weight particle size distribution diameter D90 measured by a laser diffraction-type particle size distribution measuring method of about 2.5 xcexcm or less.
[4] The particulate titanium oxide as described in 1 above, wherein the titanium oxide has a distribution constant n according to Rosin-Rammler formula is about 1.5 or more.
[5] A production process for producing particulate titanium oxide, comprising subjecting a titanium tetrachloride diluted gas obtained by diluting titanium tetrachloride to from about 10% by volume or more to about 90% by volume or less with an inert gas to high temperature oxidation with an oxidizing gas containing oxygen or steam, or both, wherein the titanium tetrachloride diluted gas and the oxidizing gas, each preheated to about 900xc2x0 C. or more, are supplied into reaction tube at a flow rate of about 20 m/sec or more and allowed to react for a time of residence at high temperatures above about 700xc2x0 C. of about 3 seconds or less.
[6] The production process as described in 5 above, wherein use is made of a titanium tetrachloride diluted gas obtained by diluting titanium tetrachloride to about 20% by volume or more and about 80% by volume or less with an inert gas.
[7] The production process as described in 5 above, wherein the temperatures for preheating the titanium tetrachloride and the oxidizing gas are each about 1,000xc2x0 C. or more.
[8] The production process as described in 5 above, wherein the titanium tetrachloride diluted gas and oxidizing gas are supplied to the reaction tube through a coaxial parallel flow nozzle having an inner tube, the inner tube having an inner diameter of about 50 mm or less.
[9] Particulate titanium oxide produced by the production method as described in 5 above.
[10] A titanium oxide composition comprising particulate titanium oxide as described in 1 above.
[11] A titanium oxide composition comprising particulate titanium oxide as described in 9 above.